The fabrication of, for example, structural members from composite materials generally involves “winding” or otherwise wrapping or applying a “green” or “prepeg” form of the composite material upon a mandrel or to other shaped tooling such as a mold, curing the thus applied composite material and then removing the shape from the tooling. Many materials and technologies exist for the production of filament winding mandrels and composite tooling in instances where production volumes or quantities are large or where cost is not an issue. More challenging, however, are the cases of limited production of prototype parts and the refinement of tooling designs during experimental programs or production troubleshooting. For example, the military has demonstrated an interest in developing tooling options for limited quantity production, depot-level maintenance and the fabrication of tooling spare parts.
The properties of such composite tooling include: 1) tailorable thermal expansion characteristics potentially matching those of the commonly used carbon-bismaleamide, invar, steel and aluminum tooling materials commonly in use; 2) compatibility with high-temperature service to enable adequate curing of a wide variety of resin systems used in composite fabrication; 3) machineability to allow on-site repair and modification; and 4) relatively low cost.
The molds used in the fabrication and curing of polymer matrix composites have been constructed from a wide variety of materials including invar, steel, aluminum, monolithic graphite, castable ceramics and carbon-epoxy and carbon-bismaleimide systems. Mold materials must exhibit high flexural and tensile strengths and durability, but perhaps most importantly, they must possess a tailorable thermal expansion to match that of the material being formed. Vacuum integrity and low heat capacity to allow relatively short heating and cooling times and thereby shorten fabrication cycles are also of vital importance for such tooling. The tooling materials of the prior art were often chosen on the basis of one or two or these desirable properties, such as strength and durability in the case of metals, at the expense of others such as tailorable thermal expansion, low heat capacity and ease of modification.
One attractive such metal mold material is Invar 36, a low carbon, 36% nickel austenitic steel that exhibits a low coefficient of thermal expansion (CTE), excellent durability and the ability to withstand high rates of thermal cycling. Its fundamental shortcomings are its low thermal conductivity and its weight. It is five times heavier than carbon-epoxy tooling of the same volume, therefore often requiring its application over lighter weight carbon-epoxy backing structures.
Other approaches to solving the composite tooling issue include electroforming a thick nickel layer over a mandrel that is subsequently removed, composite or graphite tooling over which is sprayed a metallic layer, and plastic faced plaster (PFP. Filament winding mandrels are often formed from metals, inflatable rubber bladders, or aluminum honeycombs with fiber-reinforced polymer facesheets.
One of the major difficulties with the formation of large parts is the magnification of any CTE mismatch over a large area This results in “spring-back” or “spring-in” as the formed composite part pulls away from the tool or squeezes the tool, depending upon the direction of the CTE mismatch. An excessive CTE on the female mold can cause the part to be crushed or trapped during cooling, while too low a CTE on the male tool can cause the part to lock onto the tool. An important consideration that is often ignored by mold or tooling designers is the anisotropy of composite CTE. For some polymer matrix composites, the difference in CTE between reinforcement and matrix directions can be as great as 72 ppm/° C. An often proposed solution to this issue is to lower the temperature of the cure process to minimize these differences, but this is not possible with some resin systems or practical in terms of the effect on curing time.
U.S. patent application Ser. No. 09/453,729, filed Dec. 2, 1999, now abandoned and entitled “Cellular Coal Products and Processes”, and U.S. Pat. application Ser. No. 09/902,828, filed Jul. 7, 2001, now U.S. Pat. No. 6,749,652 and entitled “Cellular Coal Products and Processes” describe coal-based cellular or porous products having a density of preferably between about 0.1 g/cm3 and about 0.8 g/cm3 that are produced by the controlled beating of coal particulate preferably up to 1 mm in diameter in a “mold” and under a non-oxidizing atmosphere. According to specifically preferred embodiments, the coal-based starting materials exhibit a “free swell index” as determined by ASTM test D720 of between about 3.5 and about 5.0. The porous products produced by these processes, preferably as a net shape or near net shape, can be readily machined using conventional techniques, adhered and otherwise fabricated to produce a wide variety of low cost, low density products, or used in their preformed shape. Such cellular products have been shown to exhibit compressive strengths of up to about 4000 psi. As described in the foregoing U.S. patent applications, the properties of such coal-based carbon foams, i.e. strength, thermal conductivity etc. can be tailored within relatively broad ranges according to the requirements of a particular application.
The application of such coal-based carbon foam materials to tooling for composite materials applications would solve most, if not all of the problems with the prior art such composite tooling materials described above.